Columbia and Yale are both very good universities, but for the public health and biostat program, Columbia is better (rank NO.5/NO.13, NO. 33/Yale doesn't have a rank in Biostat)
However, Columbia took more MS students than Yale, the class size is larger, and the students there may not as elite as at Yale. There is a reason for this, the biostat program at Yale is very small, so Yale took a small number of elite students.
which school would be more attractive to employers ... I would say Columbia seems better than Yale from what I've heard, the location is an important factor.
I have a friend who had the same situation like you last year, she visited both schools and finally chose Columbia.
BTW, I am a student from North Carolina, and also got MS admission from Columbia this year, and I will go there as a TM track student.